1. Field of the Invention
This invention relates to signal processing. Specifically, the present invention relates to signal testing.
2. Description of the Related Art
Communication systems include a transmitter communicating information and a receiver receiving information. In digital communication systems the transmitter communicates a pattern of zeros and ones to the receiver. The zeros and ones are used to represent the communicated information. Both the transmitter and receiver synchronize so that the receiver can distinguish between a logical zero and a logical one.
When high-speed digital data is transmitted over a communication channel with limited bandwidth the digital data may be degraded. One form of degradation results from the transition between digital states (e.g. zeros and ones). The transitions may be delayed by different amounts depending on surrounding data and other effects. This variation in delay of transitions is known as jitter. The variation (e.g., delay) moves the signal forward or backward. In other words, the signal (e.g. zero or one) may occur before the receiver is scheduled to sample the transition or the delay may occur after the receiver starts to sample the transition. If the receiver samples data at discrete times and jitter delay moves the transition between digital states, at a time that is different from the time expected by the receiver, the wrong data may be sampled. Improperly sampling or interpreting data in the receiver will result in an error. When the error is caused by jitter, the error is known as a jitter error.
The ability of a receiver to sample data correctly in the presence of jitter is known as jitter tolerance. The jitter tolerance is typically quantified as the peak-to-peak jitter, which is present without causing an unacceptable error rate. Measuring jitter tolerance of receivers is an important operation in the manufacture of high-speed receivers. Applying a representative signal with a known amount of jitter to a receiver and measuring the error rate is a conventional technique used to measure jitter tolerance.
As clock speeds in computers and data rates in communication systems increase, timing budgets become tighter and the need to measure and characterize jitter becomes more critical. On some networks, devices such as jitter generators may be used to generate a known amount of jitter. The jitter generator introduces jitter in a communication connection. Using these generators the jitter tolerance of the communication link (transmitter, receiver) may be determined before deploying the transmitter and receiver to the field or prior to implementing production of the transmitter or receiver.
A number of conventional techniques have been used to measure jitter tolerance. In one method a test signal is generated by clocking data with a frequency modulated clock. The clock signal is shifted to introduce errors into the signal. The signal is measured in the receiver to determine the amount of error. However, a disadvantage in this approach is the availability of clocks, which can be modulated to a large jitter, at high data rates. In addition, the modulation is symmetrical therefore the test cannot independently measure sensitivity to early and late transitions.
In a second conventional technique a test signal is filtered in a calibrated filter (e.g. low pass filter) and, optionally, the filtered signal is passed through a limiting amplifier to prevent interactions with any peaking or roll off in the receiver. However, in this approach, calibration is necessary and can be difficult; it is hard to adjust the amount of jitter; and, the modulation is symmetrical, therefore the test signal cannot independently measure sensitivity to early and late transitions.
In a third approach noise is added to a test signal with slow rise and fall times and optionally, a signal is passed through a limiting amplifier. However, in this approach, calibration is necessary and can be difficult to perform; the results of this approach is statistical in nature and therefore is hard to reproduce; and the modulation is symmetrical therefore it is difficult to measure sensitivity to early and late transitions.
Thus, there is a need in the art for a method and apparatus that measures jitter tolerance. There is a need in the art for a method and apparatus that measures jitter tolerance in high bandwidth communication links. There is a need in the art for a method and apparatus of measuring jitter tolerance that is flexible and enables the operator to easily measure a range of jitter tolerances. There is a need in the art for a method and apparatus of measuring jitter tolerance that is easily reproducible.